1. Field of the Invention
This invention concerns punch presses and more particularly a drive for a punch press ram which is coupled to punching or forming tools.
2. State of the Art
Punch presses for forming features or punching holes in sheet material workpieces involve a ram which is coupled to a forming or punching tool and drives the tool through a sheet material workpiece. Typically, the ram is driven by a crank mechanism, rotated by an AC electric motor. A flywheel may be interposed, with a clutch-brake controlling the connection of the flywheel to the crank mechanism, so that the stored energy in the rotating flywheel can be utilized to drive the ram.
A need for more sophisticated punching and forming motions has now been recognized as controlling the ram velocity to minimize punching noise, sometimes varying the velocity over the course of a punching stroke. As another example, the punch cycle time can be reduced by varying the speed of the ram over the punching cycle with a rapid ram advance and retraction combined with slow speed during actual punching.
Variable stroke control is also useful, as when forming louvers, and to minimize tooling set up.
In order to accomplish such sophisticated and flexible control over the ram motion, servo controlled hydraulic drives for rams have been developed, which allow a good deal of control over ram motion, such as variable speed and stroke punching.
See as examples of such hydraulic press ram drives, U.S. Pat. No. 4,823,658 issued on Apr. 25, 1989 for "Punch Presses", U.S. Pat. No. 4,208,935 issued on Jun. 24, 1980, for a "Control System for Hydraulic Press"; U.S. Pat. No. 5,031,431 issued on Jul. 16, 1991, for a "Method and Device for Controlling the Stroke of a Press Machine".
However, hydraulic equipment generally suffers from the disadvantages of increased cost and maintenance requirements, operates more slowly, and requires auxiliary cooling equipment.
Electrical servo motor ram drives have also been proposed, as described in U.S. Pat. No. 5,279,197 issued on Jan. 18, 1994, for a "Punch Press"; and U.S. Pat. No. 5,289,096 issued on Feb. 22, 1994, for a "Press Machine Stroke Operation Mechanism and Operation Control Method Therefor". Those patents describe a ball screw toggle mechanism for converting the rotary output of an electrical servo motor to a reciprocating ram motion.
The cost of a ball screw mechanism sufficiently sturdy to withstand the high level punching forces encountered in metal working would add substantially to the cost of a press.
The ball screw toggle mechanism described in those patents also requires a rapid motor reversal at bottom dead center, placing excessive demands on the mechanical and electrical components, and slowing the cycle time.
U.S. Pat. No. 5,279,197 also describes an electrical servo drive motor combined with a slider crank mechanism. However, in that combination, the servo motor is driven in a single direction, thus requiring the motion of a complete crank revolution to complete a punching stroke. The slider crank mechanism is also not suited to withstand the high forces required for metal working.
Accordingly, it is the object of the present invention to provide an electric servo motor punch ram drive mechanism which is capable of easily generating the high tonnage requirements for metal working, does not require rapid motor reversal when the punch is fully advanced, minimizes cycle time and improves flexibility in controlling ram velocities and stroke distances.